1. Field of the Invention
The present invention relates to an image forming apparatus and a two-dimensional light-emitting element array, and more particularly to an image forming apparatus such as a printer and a digital copier which uses a two-dimensional light-emitting element array such as a two-dimensional vertical-cavity surface-emitting laser array, a two-dimensional LED array, and a liquid crystal shutter array as a light source, in which a plurality of light-emitting elements are two-dimensionally disposed, and to a two-dimensional light-emitting element array in which a plurality of light-emitting elements such as vertical-cavity surface-emitting lasers, LED's, and liquid crystal shutters are two-dimensionally disposed.
2. Description of the Related Art
There is disclosed in, e.g., Japanese Published Unexamined Patent Application No. Hei 9-200431, an image forming apparatus which, as a light source, uses a two-dimensional light-emitting element array in which a plurality of minute light-emitting elements are two-dimensionally arranged.
FIG. 5 shows a brief configuration of the image forming apparatus which uses a two-dimensional light-emitting element array. The image forming apparatus comprises: a light source 10 configured with a two-dimensional light-emitting array; a driver circuit 12 for controlling the radiation status of the light source 10; a projection lens system 14 for guiding light from the light source 10 to the surface of a photosensitive drum 16; and a photosensitive drum 16 on the surface of which a latent image is formed by light irradiation.
The driver circuit 12 controls the radiation status of two-dimensional light-emitting elements 102 (see FIG. 4) making up the light source 10, based on image data 20 input from a storage part such as a memory or an external input part. Thereby, the light beams emitted from the light-emitting elements 102 based on the image data 20 are magnified by the projection lens system 14 and are irradiated onto the surface of the photosensitive drum 16, and a latent image conforming to the image data 20 is formed on the surface of the photosensitive drum 16.
In such an image forming apparatus, the two-dimensional light-emitting element array 120 used as the light source 10, as shown in FIG. 4, is formed by m.times.n light-emitting elements 102, in which one unit is constituted by m light-emitting elements 102 disposed at equal intervals on a reference line X, which extends in a slanting direction crossing at an angle .theta. with a reference line Y extending horizontally, and n units are disposed at equal intervals on the reference line Y.
Light from the two-dimensional light-emitting element array 120 forms n.times.m pixels 104 on the photosensitive drum, as shown in FIG. 6A, n pixels along a reference line N, which extends in the primary scanning direction, and m pixels along a reference line M, which crosses at an angle .theta. with the reference line N. (Although n pieces of reference line M can be defined for n light-emitting elements, only one is shown herein.) The magnification by the projection lens system 14 is adjusted so that the distance d between two adjacent pixels in the primary scanning direction is set to a given value determined from the resolution of the image forming apparatus.
Pixels formed on the photosensitive drum by a light-emitting element at a specific location, as shown in FIG. 6B, are formed in such a way that a pixel 104a is formed by the first irradiation and a pixel 104b is formed by the next irradiation at a position which is at a given distance d' from the pixel 104a and along a secondary scanning line on the photosensitive drum where the secondary scanning line is perpendicular to the primary scanning direction. As a result of repeating this process, m.times.n pixels 104 are arranged in the primary scanning direction. Thereby, as shown in FIG. 7, a two-dimensional image is obtained in which m.times.n secondary scanning lines are formed along the primary scanning direction.
In an image forming apparatus configured as described above, with a rotation axis defined by the optical axis of the two-dimensional light-emitting element array , if the two-dimensional light-emitting element array is installed in deviation to a position where it is rotated at angle .DELTA..theta. around the rotation axis from the proper position, pixels 104c formed on the surface of the photosensitive drum, as shown in FIG. 8A, are disposed along a reference line N1 obtained by rotation of an angle of .DELTA..theta. with respect to the normal reference line N.
Therefore, the pixel array formed on the surface of the photoconductive drum, as shown in FIG. 8B, has periodical gaps in every m pixels determined by the number of light-emitting elements in one unit in the two-dimensional light-emitting element array. If the two-dimensional light-emitting element array is installed at a position where it is rotated in the reverse direction of the foregoing, periodical overlaps occur in the pixel array with a period of m pixels.
The gap or overlap occurs in a period .LAMBDA. represented by an expression .LAMBDA.=m.multidot.d, where d is the distance between the adjacent pixels in the primary scanning direction on the photosensitive material. Hence, streak noise of spatial frequency 1/md appears on the image ultimately obtained, causing degradation of the image quality.
If the drum moves at a higher speed than the properly preset speed, pixels formed on the photosensitive drum, as shown in FIG. 9B, are positioned above the primary scanning lines along the secondary scanning lines. Conversely, if the drum moves at a lower speed than the properly preset speed, pixels formed on the photosensitive drum are positioned below the primary scanning lines along the secondary scanning lines.
Such deviation of pixel positions from the primary scanning lines is periodically repeated with a period of m pixels determined by the number of light-emitting elements in one unit in the two-dimensional light-emitting array.
Since the period of the pixel deviation is represented by the same expression .LAMBDA.=m.multidot.d, image deviation noise of spatial frequency 1/md appear as stripes on the image ultimately obtained, causing degradation of the image quality.
The foregoing description of the streak noise and image deviation noise (hereinafter referred to as image noise) has been made for a two-dimensional light-emitting element array in which a unit of light-emitting elements arranged at equal intervals on the reference line X extending in a slanting direction with respect to the reference line Y is repeatedly arranged along the reference line Y. The image noise also arise with a two-dimensional light-emitting element array in which a unit of m light-emitting elements disposed complicatedly in the vertical direction is repeatedly arranged along the reference line Y.
Therefore, in an image forming apparatus which, as a light source, uses a two-dimensional light-emitting element array in which light-emitting elements are two-dimensionally arranged, image noise of period md and spatial frequency 1/md occurs due to an installation error or a rotation speed error of the rotation drum, causing degradation of the image quality.
By the way, to make a halftone image with an image forming apparatus, mesh patterns each of which, as shown in FIG. 10, consists of q pixels in the primary scanning direction and p pixels in the secondary scanning direction are used. The period of the mesh patterns in the primary scanning direction on the surface of the photosensitive material is represented by q.multidot.d, where d is the distance between the adjacent pixels in the primary scanning direction on the surface of the photosensitive material.
If such mesh patterns are used, the presence of the above-mentioned image noise of period m.multidot.d coming from the light source causes moire fringes because of the interference between fluctuation of period q.multidot.d and fluctuation of period m.multidot.d.
The period of the moire fringes is E.multidot.d, which is determined by the least common multiple, E, of q and m. Accordingly, in addition to the above-mentioned image noise, noise of spatial frequency 1/Ed occurs, causing further degradation of the image quality.
In the related art, a method as disclosed in Japanese Published Unexamined Patent Application No. Hei 8-292384 has already been proposed. This method improves the image reproducibility by imposing some limited relationship between the number of elements and the distances between scanning lines for an apparatus which forms images by scanning multi-beam light sources having a plurality of laser elements horizontally arranged in a row.
However, when a two-dimensional light-emitting element array is used as a light source as described above, no measures are taken against image quality degradation due to periodical disturbance of a pixel array caused by a relative installation position error between the light source and a photosensitive material, or to a deviation from a setting value of rotation speed of the photosensitive material, nor against image quality degradation due to moire fringes caused by the interference between the mesh patterns set for halftone image and the above-mentioned image noise.